Ethanol as a biorefinery end product has shortcomings, including its low value and high-energy needs for extraction. Concomitantly, researchers and engineers have developed a chain-elongation platform with open cultures (reactor microbiomes) with the goal of producing medium-chain carboxylates (MCCs), such as n-caproate (C6) and n-caprylate (C8), from ethanol and short-chain carboxylates. MCCs have a higher value and are easier to extract than ethanol due to their hydrophobic characteristics when in their undissociated chemical form. The longer the chain, the more value the product will have.
Anaerobic reactor microbiomes are capable of converting organic wastes to medium chain carboxylates via a process termed reverse beta-oxidation. Generally, an electron-rich substrate, such as one containing ethanol or lactic acid, is used to drive the chain elongation of shorter chain carboxylates, such as acetate and n-butyrate, to longer chain carboxylates, such as n-caproate or n-caprylate.
Medium-chain carboxylates (MCCs, ranging from six to 12 carbons), such as n-caproate (C6), n-heptanoate (C7), and n-caprylate (C8), can be produced within the carboxylate platform by chain elongating SCCs, such as acetate (C2), propionate (C3), and n-butyrate (C4), via the reverse β-oxidation pathway by adding two carbons during each cycle. There are a variety of different processes which result in medium chain and short chain carboxylates, but the use of bioreactors and reverse beta oxidation seems particularly promising as a sustainable platform. Reverse beta oxidation is a metabolic pathway found in prokaryotes where carboxylate chains are reduced by the addition of two carbons from a substrate, the best studied of which is probably ethanol. Since chains are elongated two carbons at a time, an acetate molecule could go to butyrate, which could go to caproate, which could go to caprylate, however this progression to caprylate does not occur or results in rather low levels. Common substrates that have been demonstrated in using reverse beta oxidation include lactate, carbohydrates, and ethanol.
Clostridium kluyveri, which is the type strain for the reverse β-oxidation pathway, has been well studied to ascertain the mechanistic understanding of this pathway. In several studies with microbiomes the main product has been the even-chain MCC n-caproate, while small amounts of n-caprylate have been co-produced. Reactor microbiomes have also produced the uneven-chain MCC n-heptanoate, albeit at a lower selectivity (product vs. consumed substrate) than for n-caproate production. MCCs can be utilized as antimicrobial agents in agriculture; intermediates for fragrances and flavors; and precursors for renewable diesel fuel and aviation fuel. In all of these markets, a premium is available for longer-chain products (e.g. n-caprylic acid vs. n-caproic acid) due to their increased hydrophobicity and energy density.
Until now, the literature has described chain elongation processes to work by elongating dilute ethanol and acetate into n-caproate (6-carbons; C6), but with n-caprylate as a minor component side product, if produced at all. The value of n-caprylate is about twice that of caproate on a weight basis. Since n-caprylate is considerable more valuable than n-caproate by weight, the question was raised whether mainly n-caprylate could be produced.